A construction machine such as power shovel or the like is equipped with a variable displacement type hydraulic pump adapted to be driven by an engine.
A hitherto known apparatus for controlling a variable displacement type hydraulic pump has a function of properly controlling an inclination angle of a swash plate in the pump to assure that an output torque from the engine matches with an absorption torque absorbed by the pump at all times in order to effectively utilize the output torque from the engine.
However, the conventional apparatus has a drawback that an improvement effect covering a fuel consumption characteristic of the engine and a pump efficiency can not be expected due to the fact that the apparatus is intended to control only the variable displacement type hydraulic pump.
On the other hand, an apparatus for varying an absorption torque absorbed by a variable displacement type hydraulic pump in dependence on a given operation mode (operation to be performed under a high intensity of load, operation to be performed under a low intensity of load or the like) was already proposed by a Japanese Laid-Open Patent No. 204987/1985.
However, the last-mentioned conventional apparatus has a drawback that it can deal with only a problem in respect of such a state that the engine is excessively heated. Incidentally, it is thinkable as a countermeasure to be taken at the time when the engine is excessively heated that an output horsepower from the engine and the number of revolutions of the engine are reduced. However, when this countermeasure with which an absorption horsepower absorbed by the pump which is a direct load exerted on the engine does not vary is employed, it not only takes a long time until a normal operational state is restored from the state that the engine is excessively heated, resulting in a satisfactory operation failing to be performed, but also a running time of the engine is shortened.
Further, the conventional apparatuses detect a pressure of hydraulic oil delivered from the pump with the use of pressure detecting means in order to control an inclination angle of a swash plate in the pump, but there arises such a problem that operation of the engine is interrupted or an output torque from the engine fails to be transmitted to the pump when an abnormality relative to the pressure detecting means occurs, because they can not entirely deal with the above-mentioned abnormality.
An object of the present invention is to provide an apparatus for controlling a hydraulic pump which assures that a fuel consumption cost required for an engine can be reduced and moreover an operational efficiency of the hydraulic oil can be improved substantially.
Other object of the present invention is to provide an apparatus for controlling a hydraulic oil which assures that a normal operational state of the engine can be restored when the engine is excessively heated.
Another object of the present invention is to provide an apparatus for controlling a hydraulic pump which assures that the pump can be operated even at the time when means for detecting a pressure of hydraulic oil delivered from the pump becomes abnormal in function.